Recombinant protein expression has become a major tool in molecular biology. Recombinant expression of genes from transformed organisms is now a non-dispensable method for production of proteins for various purposes, such as protein characterization, protein identification, protein function and structure study, etc. Recombinant expression of genes has also been utilized for large scale production of proteins for commercial purposes, such as, to be used as enzymes, nutritional proteins, biopharmaceuticals (drugs), etc.
A recombinant DNA encoding a protein (polypeptide) can be introduced into a “host” cell where it is expressed and translated into the protein, which is stored inside the cell, e.g., in the cytoplasm or cytosol of the cell, and subsequently isolated or purified from the host cell. However, this procedure has some disadvantages. For example, the recombinant polypeptide may be toxic to the host cell and may even kill the host cell as the polypeptide is continuously synthesized in the cell and begins to accumulate. The recombinant polypeptide may also be degraded by intracellular proteases or be subject to unwanted post-translational modification, such as acetylation, etc. In addition, as the intracellular concentration of the recombinant polypeptide increases, the host cell machinery may slow down or cease manufacturing via a “feedback mechanism” directing termination of polypeptide synthesis, see, e.g., U.S. Pat. No. 5,470,719.
Exporting or secreting a recombinant protein outside the cell can overcome the problems associated with the accumulation of the protein in the cell. Furthermore, because the protein can be harvested fairly easily from the chemically much simpler extracellular environment, it is preferred for the recombinant protein to be secreted into the extracellular environment for easy downstream processing.
There remains a need for enhanced expression and secretion of proteins from the cells.